Information and Specification Reference for PTFE Resin Compounds

This information bulletin is a source of general information for the most commonly used PTFE resin compounds, and the values listed are intended only to inform potential users about the practical limitations inherent in these engineering materials, While the range of engineering property values covers the various methods of fabrication and testing normally employed, the method of fabrication greatly affects the property obtainable in PTFE resins. Consequently, these values must not be extracted for use in writing specifications.

EXPLANATORY NOTES

Quality Control

The density, tensile strength, elongation, porosity and dielectric strength values are commonly used for quality control of PTFE resin compounds.

Anisotropic Properties

The mechanical properties of PTFE compounds are highly directional; generally, those measured parallel to the direction of molding are lower than the values measured perpendicular to the molding direction.

Dielectric Strength

The dielectric strength value (volts/mil) of PTFE increases as the thickness of the test specimen is decreased, but compounded PTFE resin materials can give unexpectedly low values either because of the electrical conductivity of the filler or the presence of voids. Since PTFE resin compounds are mixtures of Materials with no chemical bonding, voids, no matter how small, are always present, and are often exaggerated during preparation of the sample. High humilities can also reduce the dielectric strength, as can spillage of conducting liquids, electrolytes or greases.

Hardness

The hardness values given are based on initial readings as described in ASTM D 2240-64T. However, the hardness measurement is not sufficiently definitive or reproducible enough for use as an index of quality for PTFE resin products.

Coefficient of Friction

The PTFE material coefficient of friction is highly dependent on the load, the speed and the method of testing. Generally, high loads and low speeds give low coefficient values; low loads and high speeds produce high values for the coefficient of friction, the values in this bulletin are based on thrust washer test specimens.

Total Deformation

As with all thermoplastics, the properties of PTFE resins and their compounds are dependent on time, temperature and test method. This behavior is particularly true with total deformation. In the case of PTFE resins, this property also depends on the fabrication technique. Therefore, the values listed must be regarded as a simple summary of a very complex subject.

Coefficient of Thermal Expansion

It is important to note that true linear thermal expansion can be measured only on material from which all stresses have been removed. Changes in the dimensions of a particular part due to changes in temperature can exceed the true thermal expansion if stresses are present.

PV Value

The PV value of a compound is the product of the unit load P (psi) on the projected area and the surface velocity V (fpm). Intermittent ser- vice, reciprocating motion, special cooling and design innovations sometimes permit PTFE materials to operate at higher PV values than the limit established using continuous rotary motion.

K Factor

The K factor is a wear factor which indicates the relative rate of wear for PTFE materials run under identical conditions. K factor values are calculated from empirically determined limiting PV values using the formula R=KPVT where R=radical wear (in), P=unit load (psi), V=surface velocity (fpm), and T =time (hr).

Wear

There is increasing evidence that wear is not directly related to a PTFE compound's other properties. It is wrong, therefore, to assume that a compound having a relatively low coefficient of friction or high thermal conductivity necessarily has better wear properties.

Factors known to affect the wear rate of a given compound include load, speed, temperature, mating material and its surface finish, and environment. While PV values and K factors provide a general guide for design purposes, practical tests should still be carried out wherever possible.

Wear resistance (wear when rubbing against smooth surfaces) must not be confused with abrasion resistance (wear when rubbing against abrasive surfaces). In general, the addition of fillers to PTFE resins improves wear resistance but reduces abrasive resistance by providing discontinuities in the PTFE resin which can be entered by sharp practices that may tear the material.

Effect of Filler Content on Property Values

In contrast to their effect on other plastics, fillers do not reinforce the PTFE resins. There is essentially no bonding achieved between the PTFE resin and the filler particles. In fact, the filler particles interfere with the PTFE-to- PTFE bonding or coalescence and, therefore, actually lower some physical properties. The primary functions of the filler in PTFE resins are (1) to hinder the relative movement of PTFE molecules past one another and in this way reduce creep or deformation of the part, (2) to reduce the wear rate of parts used in dynamic applications, and (3) to reduce the coefficient of thermal expansion.

Moisture Absorption

Unfilled PTFE does not absorb water. Filled PTFE compounds absorb small amounts of moisture. Since PTFE resin and fillers are not hygroscopic, any moisture picked up simply fills the voids. Extent of pickup is so small that the dimensional stability is essentially unaltered.

Temperature Resistance

Filled PTFE compounds have been used successfully over the full range of continuous service temperature associated with unfilled PTFE (-450°F to +500°F) and intermittently at higher temperatures. Fillers help strengthen PTFE compounds to elevated temperatures. This may extend the useful application range of the material, particularly when compressive stress is a consideration.

Chemical Resistance

Unfilled PTFE is chemically inert to attack by most active chemicals except alkali metals and certain fluorinated materials at high temperature and pressure. However, where filled PTFE materials are used for corrosive service, the filler must not be susceptible to attack by the contacting material. Glass fiber, carbon and graphite fillers are usually used in corrosive services.

Electrical Properties

With many applications, the electrical properties of unfilled PTFE are superior to those required; a slight reduction of these values by the induction of fillers often can be tolerated. Relatively non- conductive fillers such as glass fiber, mica or molybdenum disulfide have the least effect on electrical properties of unfilled PTFE. Conductive fillers such as carbon, graphite and bronze render the compounds unsuitable for most electrical applications. Because the range of filler materials suitable for compounding with PTFE is very broad, electrical properties can generally be varied as needed.

Basic Fillers Used in PTFE Resins

Glass Fiber is the most universally used PTFE filler. Since glass has the least effect on chemical and electrical properties and adds greatly to mechanical properties of unfilled PTFE, glass-fiber compounds afford the best balance of chemical, electrical and mechanical properties of any filled PTFE compounds.

Bronze compounds have higher hardness, lower wear, higher comprehensive strength, better dimensional stability, higher thermal conductivity, lower creep and cold flow than most other compounds. However, test data shows that bronze compounds are not suited to many electrical applications or to those that involve corrosive service environments.

Molybdenum Disulfide adds substantially to the hardness, stiffness and wear resistance of PTFE resins. It reduces starting friction and has little effect on PTFE's electrical and chemical properties. Generally, only small amounts of molybdenum disulfide are used, most often in conjunction with complementary filler.

How to Choose and Specify a Compounded PTFE Part

The tables that follow show the variety of standard filled compositions that are available. The selection of a filled PTFE resin part for a particular application can be broken into four sequential steps.

Define the job or functional requirements of the part. This definition should include such requirements as physical strength, electrical properties, flexibility, load-bearing, availability of lubrication, operating speeds, continuity of motion, environmental temperature ranges, atmospheres and corrosive fluids, dimensional tolerances and tolerable changes.

Select the PTFE resin compound to be used. The listings in the tables can be used to reduce the potentially feasible compounds to two or three based on the functional requirements.

In conjunction with Jrlon engineering, analyze the potential filled compounds and the means of fabrication from a technical and cost-effectiveness perspective.

Having selected the best filled PTFE compound and the most appropriate means of fabrication, set meaningful specification values which define a quality PTFE resin product or part.

The data shown should not be used for specifications and the ranges of data listed are not intended to establish limiting property values. While the lower values represent the minimum properties recommended (generally given by ram extruded materials), the higher values often are attainable only by special fabrication techniques. Since the property values of compounded PTFE vary considerably with the method of fabrication, specifications must be arranged by mutual agreement between the purchaser and the fabricator.